Isomerization units are important to refineries for providing a simple process of upgrading traditional low octane gasoline blending stocks. Additionally, refineries are able to regulate benzene content by hydrogenation of the benzene fraction. A naphtha hydrotreating (NHT) and isomerization unit 100 is comprised of multiple conventional distillation columns as shown in FIG. 1. A feed stream from an NHT reactor is sent to a first stabilizer column 102. The first stabilizer column 102 removes non-condensable components from the feed stream as off-gas. A stabilized bottoms product stream is fed from the first stabilizer column 102 to a naphtha splitter column 104. The naphtha splitter column 104 separates the stabilized bottom product into a light naphtha overhead stream and heavy naphtha bottoms stream.
The light naphtha overhead stream is comprised mainly of C5-C6 components. Typically, the light naphtha overhead stream is fed to a deisopentanizer column 106, which concentrates i-C5 as an overhead stream of the deisopentanizer column 106. The remaining C5-C6 components are obtained as a bottoms stream of the deisopentanizer column 106 and are fed to an isomerization reactor 108 for octane upgrading. Following isomerization, unstable isomerate from the isomerization reactor 108 is further processed in a second stabilizer column 110. Subsequently, light hydrocarbons are removed in an overhead stream as off-gas and stable isomerate is sent to a depentanizer column 112 to remove a concentrated stream of C5 components.
A C5 rich stream from the depentanizer column 112 is recycled back to the deisopentanizer column 106 upstream to remove i-C5 product. The bottoms product stream from the depentanizer column 112 is fed to a deisohexanizer column 114. A C6 isomerate product stream is removed from the deisohexanizer column 114 as an overhead stream and a heavy isomerate product stream (mainly C7+ cut) is removed from the deisohexanizer column 114 as bottoms stream. An n-C6 rich cut is removed as a side cut from the deisohexanizer column 114 and is recycled to the isomerization reactor.
The prior art system 100 of FIG. 1 possesses several disadvantages. The first and second stabilizer columns 102, 110 in the prior art system 100 operate at relatively high pressures (˜100 psig and ˜160 psig respectively in this case). Due to the partial condensation used in conventional stabilizers, liquid losses to off gas are high. Similarly, the naphtha splitter column 104 also operates at a high pressure (˜75 psig). Consequently, the column reboilers operate on moderately expensive medium pressure (MP) steams.
Furthermore, due to the number of columns involved, typical high operating temperatures, and high operating pressures observed in the various stages of the process, a conventional isomerization unit can be a costly and energy-intensive operation. With demand for isomerization units ramping up in the refining industry, improving the process scheme to make it less costly and more efficient is desirable.